This invention relates to methods utilizing nuclear magnetic resonance (NMR) techniques for imaging fluid flow. The invention has particular applicability, but is not limited, to the measurement of blood flow in medical diagnostic studies.
By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons and/or neutrons. Due to the spin of the protons and the neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is paced in a static, homogeneous magnetic field, B.sub.o, a greater number of nuclear magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B.sub.o, the magnetic moments precess about the axis of the field at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, .omega., also referred to as the Larmor frequency, is given by the equation .omega.=.gamma.B, in which .gamma. is the gyromagnetic ratio which is constant for each NMR isotope and wherein B is the magnetic field acting upon the nuclear spins. It will be thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned.
The orientation of magnetization M, normally directed along the magnetic field B.sub.o, may be perturbed by the application of magnetic fields oscillating at the Larmor frequency. Typically, such magnetic fields designated B.sub.1 are applied orthogonal to the direction of the static magnetic field by means of a radio-frequency (RF) pulse through coils connected to a radio-frequency transmitting apparatus. The effect of field B.sub.1 is to rotate magnetization M about the direction of the B.sub.1 field. This may be best visualized if the motion of magnetization M due to the application of RF pulses is considered in a Cartesian coordinate system which rotates at a frequency substantially equal to the resonant frequency about the main magnetic field B.sub.o in the same direction in which the magnetization M precesses. In this case, the B.sub.o field is chosen to be directed in the positive direction of the Z axis, which, in the rotating Cartesian system, is designated Z' to distinguish it from the fixed-coordinate system. Similarly, the X and Y axes are designated X' and Y'. Bearing this in mind, the effect of an RF pulse, then, is to rotate magnetization M, for example, from its direction along the positive Z' axis toward the transverse plane defined by the X' and Y' axes. An RF pulse having either sufficient magnitude or duration to rotate magnetization M into the transverse plane (i.e., 90.degree. from the direction of the B.sub.o field) is conveniently referred to as a 90.degree. RF pulse. Similarly, if either the magnitude or the duration of an RF pulse is selected to be twice that of a 90.degree. pulse, magnetization M will change direction from the positive Z' axis to the negative Z' axis. This kind of an RF pulse is referred to as a 180.degree. RF pulse, or for obvious reasons, as an inverting pulse. It should be noted that a 90.degree. or a 180.degree. RF pulse will rotate magnetization M through the corresponding number of degrees from any initial direction of magnetization M. It should be further noted that an NMR signal will only be observed if magnetization M has a net transverse component (perpendicular to B.sub.o) in the transverse plane. A 90.degree. RF pulse produces maximum net transverse magnetization in the transverse plane since all of magnetization M is in that plane, while a 180.degree. RF pulse does not produce any transverse magnetization.
RF pulses may be selective or nonselective. Selective pulses are typically modulated to have a predetermined frequency content so as to excite nuclear spins situated in preselected regions of the sample having precession frequencies as predicted by the Larmor equation. The selective pulses are applied in the presence of localizing magnetic-field gradients. Nonselective pulses generally affect all of the nuclear spins situated within the field of the RF pulse transmitter coil and are typically applied in the absence of localizing magnetic field gradients.
There are two exponential time constants associated with longitudinal and transverse magnetizations. The time constants characterize the rate of return to equilibrium of these magnetization components following the application of perturbing RF pulses. The first time constant is referred to as the spin-lattice relaxation time (T.sub.1) and is the constant for the longitudinal magnetization to return to its equilibrium value. Spin-spin relaxation time (T.sub.2) is the constant for the transverse magnetization to return to its equilibrium value in a perfectly homogeneous field B.sub.o. In fields having inhomogeneities, the time constant for transverse magnetization is governed by a constant denoted T.sub.2 *, with T.sub.2 * being less than T.sub.2. In some instances, it is desirable to rapidly dissipate transverse magnetization component by applying a magnetic field gradient, as will be described more fully hereinafter.
There remains to be considered the use of magnetic field gradients to encode spatial information (used to reconstruct images, for example) into NMR signals. Typically, three such gradients are necessary: EQU G.sub.x (t)=.differential.B.sub.o /.differential.x, EQU G.sub.y (t)=.differential.B.sub.o /.differential.y, and EQU G.sub.z (t)=.differential.B.sub.o /.differential.z.
The G.sub.x, G.sub.y, and G.sub.z gradients are constant throughout the imaging slice, but their magnitudes are typically time dependent. The magnetic fields associated with the gradients are denoted, respectively, b.sub.x, b.sub.y, and b.sub.z, wherein EQU b.sub.x =G.sub.x (t)x, EQU b.sub.y =G.sub.y (t)y, and EQU b.sub.z =G.sub.z (t)z
within the volume.
The NMR phenomenon has been utilized by structural chemists to study in vitro the molecular structure of organic molecules. More recently, NMR has been developed into an imaging modality utilized to obtain transaxial images of anatomical features of live human subjects, for example. Such images depicting nuclear-spin distribution (typically protons associated with water in tissue) spin lattice (T.sub.1), and/or spin-spin (T.sub.2) relaxation constants are of medical diagnostic value in determining the state of health of tissue in the region examined. NMR techniques have also been extended to in-vivo spectroscopy of such elements as phosphorus and carbon, for example, providing researchers with the tools for the first time to study chemical processes in a living organism. Equally important is the use of NMR as a non-invasive modality to study the direction and velocity of blood flows. Blood flow studies typically rely on NMR signals produced by protons associated with water molecules contained in blood fluid. It is with the flow measurement application of NMR that the present invention is concerned.
Most conventional flow imaging techniques are either based on time-of-flight principles, phase encoding, or modulation of the free precession frequency due to flow along a gradient during the free precessiuon interval. These techniques are described respectively by I. R. Young, et al, Am. J. Roentgenol. Vol. 137, p. 895 (1981); P. R. Moran, Mag. Res. Img., Vol. 1, pp. 197-203 (1982); and H. A. Lent, et al, Second Annual Meeting of The Society of Magnetic Resonance in Medicine, San Francisco, Aug. 16-19, 1983, Abstract page 211 in Book of Abstracts. Semi-quantitative flow studies have also been conducted by making use of the standard spin-echo sequence in which the signal intensity in the presence of fluid flow is reduced due to a combination of dephasing effects occurring during the interpulse interval (i.e., the time between the 90.degree. excitation and the 180.degree. inverting pulses). The semi-quantitative techniques have also taken advantage of the dephasing effects occurring during the time between successive repetitions of the pulse sequence. Another effect utilized in semi-quantitative flow studies has its origin in a reduction of the spin-echo signal amplitude due to the motion of nuclear spins in the presence of a magnetic field gradient, as first described by Carr and Purcell, Physics Rev., Vol. 94, p. 630 (1954). The method for visualization of in-plane flow in accordance with the invention makes use of the latter phenomenon in a unique and unobvious manner.
It is one object of the invention to provide a method for reconstructing pure-flow images in which contributions from stationary nuclear spins are removed by cancellation.
It is another object of the invention to provide a method for reconstructing flow images which is compatible with conventional multiple echo proton image reconstruction techniques.